Solar ultraviolet (UV) radiation reaching the earth is a combination of UVB (290-320 nm) and UVA (320-400 nm) wavelengths. Since UVA is less energetic than UVB, UVB has long been thought to be the factor responsible for the damaging effects of solar radiation. But with modern tools such as in vitro models, it has been proven that UVA plays a major role. The objective of this review is to show how skin may be exposed to UV light and to highlight the clinical aspects of UV-induced skin damages with the respective contribution of UVB or UVA. Even if UVA is less energetic than UVB, it is more abundant and penetrates deeper into the skin, reaching as far as the dermis. Various factors also influence skin exposure to UV light: the latitude, season, and time of the day. Acute as well as chronic sun exposure induces short- and long-term clinical damages. Erythema and pigmentation are immediate responses of normal human skin exposed to UV radiation. The long-term effects are photoaging and photocarcinogenesis. In particular, UVA appears to play a major role in the deterioration of dermal structure leading to the photoaged appearance of the skin.

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Cutaneous damages such as sunburn, pigmentation, and photoaging are known to be induced by acute as well as repetitive sun exposure. Not only for basic research, but also for the design of the most efficient photoprotection, it is crucial to understand and identify the early biological events occurring after ultraviolet (UV) exposure. Reconstructed human skin models provide excellent and reliable in vitro tools to study the UV-induced alterations of the different skin cell types, keratinocytes, fibroblasts, and melanocytes in a dose- and time-dependent manner. Using different in vitro human skin models, the effects of UV light (UVB and UVA) were investigated. UVB-induced damages are essentially epidermal, with the typical sunburn cells and DNA lesions, whereas UVA radiation-induced damages are mostly located within the dermal compartment. Pigmentation can also be obtained after solar simulated radiation exposure of pigmented reconstructed skin model. Those models are also highly adequate to assess the potential of sunscreens to protect the skin from UV-associated damage, sunburn reaction, photoaging, and pigmentation. The results showed that an effective photoprotection is provided by broad-spectrum sunscreens with a potent absorption in both UVB and UVA ranges.

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Photodermatoses are a group of disorders resulting from abnormal cutaneous reactions to solar radiation. They include idiopathic photosensitive disorders, drug or chemical induced photosensitivity reactions, DNA repair-deficiency photodermatoses and photoaggravated dermatoses. The pathophysiology differs in these disorders but photoprotection is the most integral part of their management. Photoprotection includes wearing photoprotective clothing, applying broad spectrum sunscreens and avoiding photosensitizing drugs and chemicals.

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Skin exposure to acute or repetitive ultraviolet light induces risks which are now well identified. An efficient photoprotection is thus required for both UVB and UVA radiation. In particular, increasing evidence of the detrimental effects of UVA on skin has led to the development of a new generation of sunscreens that provide effective protection throughout the whole UV radiation spectrum. Many new UV filters have been introduced in the last decade, particularly UVA filters, with improved efficacy and safety. Sunscreen filters must be carefully combined to achieve esthetically pleasing products offering photostable and well-balanced photoprotection.

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Skin exposure to sunlight can cause many adverse effects. It is now recognized that both Ultraviolet A (UVA) and UVB wavelengths are responsible for the detrimental effects of solar radiation on skin. With our increasing knowledge on the harmful effects of UVA, the need for effective, well-balanced photoprotection has become more crucial. Numerous clinical studies showed that well-balanced sunscreen, with a SPF/UVAPF ratio ≤ 3, provide the most effective protection against pigmentation (especially on dark skin), DNA damage, UV-induced skin immunosuppression and photodermatoses. The calculation of UVA protection required in Asia revealed its particular importance in India, and gives clear evidence that the SPF value alone is not sufficient to evaluate the efficacy of a sunscreen.

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